The present invention concerns a method of using the BK enhancer in the presence of an immediate-early gene product of a large DNA virus to increase transcription of a recombinant gene in eukaryotic host cells. The BK enhancer is a defined segment of DNA that consists of three repeated sequences (the prototype BK enhancer is depicted in Example 17, below). However, a wide variety of BK enhancer variants, not all consisting of three repeated sequences, are known in the art and suitable for use in the invention.
The BK enhancer sequence exemplified herein is obtained from BK virus, a human papovavirus that was first isolated from the urine of an immunosuppressed patient. BK virus is suspected of causing an unapparent childhood infection and is ubiquitous in the human population. Although BK virus grows optimally in human cells, the virus undergoes an abortive cycle in non-primate cells, transforms rodent cells in vitro, and induces tumors in hamsters. BK virus is very similar to SV40, but the enhancer sequences of the two papovaviruses, SV40 and BK, differ substantially in nucleotide sequence. The complete nucleotide sequence of BK virus (.about.5.2 kb) has been disclosed by Seif et al., 1979, Cell 18:963, and Yang and Wu, 1979, Science 206:456. Prototype BK virus is available from the American Type Culture Collection (ATCC), 12301 Parklawn Dr., Rockville, Md. 20852-1776, under the accession number ATCC VR-837. A restriction site and function map of prototype BK virus is presented in FIG. 1 of the accompanying drawings.
Enhancer elements are cis-acting and increase the level of transcription of an adjacent gene from its promoter in a fashion that is relatively independent of the position and orientation of the enhancer element. In fact, Khoury and Gruss, 1983, Cell 33:313, state that "the remarkable ability of enhancer sequences to function upstream from, within, or downstream from eukaryotic genes distinguishes them from classical promoter elements . . . " and suggest that certain experimental results indicate that "enhancers can act over considerable distances (perhaps &gt;10 kb)."
The present invention teaches that unexpected increases in transcription result upon positioning the BK enhancer immediately upstream of (on the 5' side of) the "CAAT" region of a eukaryotic promoter that is used in tandem with the BK enhancer to transcribe a DNA sequence encoding a useful substance. The CAAT region or "immediate upstream region" or "-80 homology sequence" is a cis-acting upstream element that is a conserved region of nucleotides observed in promoters whose sequences for transcriptional activity have been dissected. The CAAT region is found in many, but not all, promoters. In other promoters, equivalent cis-acting upstream elements are found, including SP1 binding sites, the octa sequence, nuclear factor 1 binding sites, the AP1 and AP2 homologies, glucocorticoid response elements, and heat shock response elements. The CAAT region equivalent in the adenovirus major late promoter is the upstream transcription factor (UTF) binding site (approximate nucleotides -50 to -65 upstream of the CAP site). The CAAT sequence mediates the efficiency of transcription and, with few exceptions, cannot be deleted without decreasing promoter strength.
Enhancer elements have been identified in a number of viruses, including polyoma virus, papilloma virus, adenovirus, retrovirus, hepatitis virus, cytomegalovirus, herpes virus, papovaviruses, such as simian virus 40 (SV40) and BK, and in many non-viral genes, such as within mouse immunoglobulin gene introns. Enhancer elements may also be present in a wide variety of other organisms. Host cells often react differently to different enhancer elements. This cellular specificity indicates that host gene products interact with the enhancer element during gene expression.
Enhancer elements can also interact with viral gene products present in the host cell. Velcich and Ziff, 1983, Cell 40:705; Borrelli et al., 1984, Nature 312:608; and Hen et al., 1985, Science 230:1391, disclose that the adenovirus-2 early region 1A (E1A) gene products repress activation of transcription induced by the SV40, polyoma virus, mouse immunoglobulin gene and adenovirus-2 E1A enhancers. Eukaryotic expression vectors that utilized enhancers to increase transcription of recombinant genes consequently were not expected to work better than vectors without enhancers in E1A-containing host cells. In striking contrast to the prior art methods of using enhancers, the present method for using the BK virus enhancer element involves using the E1A gene product or a similar immediate-early gene product of a large DNA virus to maximize gene expression. Thus, the present invention teaches that the ability of the BK enhancer to promote transcription of DNA is increased in the presence of the E1A gene product of any adenovirus.
The E1A gene product (actually, the E1A gene produces two products, which are collectively referred to herein as "the E1A gene product") is an immediate-early gene product of adenovirus, a large DNA virus. The present invention encompasses the use of any immediate-early gene product of a large DNA virus that functions similarly to the E1A gene product to increase the activity of the BK enhancer. The herpes simplex virus ICP4 protein, described by DeLuca et al., 1985, Mol. Cell. Biol. 5: 1997-2008, the pseudorabies virus IE protein, described by Feldman et al., 1982 P.N.A.S. 79:4952-4956, and the E1B protein of adenovirus are all immediate-early gene products of large DNA viruses that have functions similar to the E1A protein. Therefore, the method of the present invention includes the use of the ICP4, IE, or E1B proteins, either in the presence or absence of E1A protein, to increase the activity of the BK enhancer.